1. Field of the Invention
This invention relates to an assembly for accessing an information recording medium, i.e , to an accessing device with a transducer for an apparatus for recording and/or reproducing information on a recording medium.
2. Description of Prior Art
In a recording and/or reproducing apparatus such as an electronic still camera, a magnetic disc apparatus, an optical disc apparatus, etc., there is provided a transducer, such as a magnetic head, a photodetector, etc., for recording and/or reproducing information on a recording medium, and various accessing devices with transducers have been proposed with mechanisms for shifting the transducer from track to track on the medium. As illustrated in FIG. 4, a transducer accessing device of this type includes a magnetic head as a transducer 1, a movable transducer carriage 2 for supporting the transducer 1, a motor 3, and a motion converting element 4 such as a disc cam or a cylindrical cam driven by the motor 3 to provide the carriage 2 with a predetermined displacement. In the illustrated device, the rotational motion of the motor 3 is converted to linear motion in the direction of movement of the carriage 2, i.e., in the radial direction of a disc-shaped recording medium 6, by a cam 4 and a cam follower 5. Therefore, in response to the rotation of the motor 3, the transducer 1 supported by the carriage 2 moves between a plurality of tracks 7 formed on the recording medium 6. Conventionally, the motion converting cam element has been of the type that the distance from its axis to its peripheral surface, which rollingly contacts the cam follower, increases linearly in proportion to the angle of rotation from a reference position. Thus the relationship between the quantity of displacement of the transducer 1 and the angle of rotation of the motor 3, hence the cam 4, is linear, as shown in FIG. 5.
Stepping motors have been widely used in implementing precision motions because stepping motors can be controlled relatively precisely by open loop control systems. However, when a stepping motor is used for the motor 3 in the above-described device, the following problem arises: as shown in FIG. 6, the relationship between the number of pulses supplied to the stepping motor and the corresponding angle of rotation is not an accurate stepwise relationship, but is represented by a curve including superimposed transient ripple components. Therefore the carriage 2 does not stop at a predetermined position corresponding to the number of supplied pulses and it is difficult to position the transducer 1 correctly with respect to a desired track on the medium 6.
In order to avoid this drawback, Japanese Laid-Open Patent Publication No. 3754/1984 has proposed that the cam 4, which has a smooth peripheral surface for providing linear characteristics as shown in FIG. 5, is replaced by a cam as shown in FIG. 8 having stepwise shifting characteristics as shown in FIG. 7. When the transducer is at a predetermined track position, the movement of the carriage is interrupted to prevent unnecessary movement due to the ripple rotational motion of the motor from being transmitted to the transducer.
However, when a motion converting element (cam) having the characteristics shown in FIG. 7 is used, a new problem arises since in the range where no displacement occurs in response to the rotation of the motor (hereinafter called "pause range"), the position of the transducer will not change even if the angle of rotation of the motor is slightly varied. In an apparatus of this type, when information is reproduced by the transducer from a track formed on the medium, so-called tracking control is necessary for finely controlling the position of the transducer, for example, in response to the envelope of reproduced signals obtained by the head 1. As stated above, however, when the rotational position of the motor is in the pause range as shown in FIG. 7, fine control of the transducer position is impossible, and tracking control is accordingly difficult to achieve. On the other hand, when the characteristics of the motion converting element are selected as shown in FIG. 9 such that the output displacement changes with very small steps in a generally linear relationship with the number of pulses supplied to the motor, a relatively precise tracking control is possible. (In FIG. 9, the unit of the horizontal axis is the number of the steps of rotation of the stepping motor representing the angular position of the cam and the unit of the vertical axis is the track number on the medium representing the output displacement). In this case, however, many input pulses must be supplied to shift the transducer from one track (the n-th track) to the next track (the n+1-th track) Accordingly, the speed at which the information bearing tracks are accessed via the transducer is greatly reduced. Low speed of transducer accessing is a serious drawback for a device in the storage portion of a recording and reproducing apparatus.